Interactions between spiral galaxies and their dwarf satellites are often spectacular, producing extended streams of stripped gas and triggering new generations of star formation. The most striking local example of such an interaction lies in the outer halo of the Milky Way in the form of the Magellanic Stream. Extending for over 140 degrees across the Southern Sky, the Stream is a giant ribbon of gas trailing the orbit of the Large and Small Magellanic Clouds as they journey around the Galaxy. Since its discovery over 40 years ago, the Stream has puzzled observers and theorists alike and raised many questions. How was it physically removed from the Magellanic Clouds? Did it originate in the LMC or SMC? And what will its ultimate fate be? New spectroscopic observations with the Hubble Space Telescope and the Very Large Telescope are addressing these questions and finding the origin of the Stream to be surprisingly complex.

Figure 1 : Top: In this combined all-sky radio and visible-light image, the Magellanic Stream is shown in pink. The radio observations, taken from the Leiden-Argentine-Bonn (LAB) Survey, have been combined with a visible-light panorama. The Milky Way is the light blue band in the center of the image. The brown clumps are interstellar dust clouds in our galaxy. The Magellanic Clouds are seen in white at bottom right. Bottom: close-up of the Stream with our HST/COS sightlines marked with crosses. Credit: NASA, ESA, D. Nidever et al., NRAO/AUI/NSF, A. Mellinger, LAB Survey.

Measuring the chemical abundance (metallicity) of interstellar gas clouds requires finding UV-bright background sources, such as quasars. By splitting the quasar light into its constituent colors, the absorption lines imprinted by foreground gas clouds can be measured. These lines encode detailed information on the chemical composition and motion of the foreground clouds. Using observations from the Cosmic Origins Spectrograph (COS) installed on Hubble in 2009, we observed eight active galactic nuclei (AGN) lying behind or near the Stream. By comparing the strength of the neutral oxygen (O I) and ionized sulfur (S II) UV absorption lines to the strength of the atomic hydrogen (H I) 21 cm emission measured by radio telescopes, we derived the Stream’s metallicity in each direction. O I and S II were chosen for these measurements since they are largely unaffected by ionization and dust-depletion effects, so their ratios with H I provide robust metallicity indicators.

We found the Stream’s metallicity to be only ≈10% of the solar value in three separate directions sampling most of its length, considerably lower than the current-day average metallicity of the SMC (≈20% solar) and the LMC (≈50% solar). However, the age of the Stream is estimated from tidal models to be around 2 billion years, and information on the metallicity evolution of the Magellanic Clouds indicates that 2 billion years ago, the SMC abundance was ~10% solar, matching the value we measure in the Stream, whereas the LMC abundance was much higher, at ~30-40% solar. Our results thus support a scenario in which most of the Stream was stripped from the SMC (not the LMC). It has not self-enriched since its formation, because there is no evidence for ongoing star formation in the gas. In a sense, we have measured a fossil record of the Stream at the time of its birth in the SMC about 2 billion years ago.

However, a fourth sightline we studied (toward the AGN Fairall 9) tells a very different story. In this direction, which lies close to the Magellanic Clouds on the sky, the sulfur abundance in the Stream is found to be 50% solar, five times higher than the value measured in the other directions, and much higher than expected for gas that has been stripped from the SMC. Furthermore, the Fairall 9 direction intercepts a filament of the Stream that appears to connect kinematically to the LMC. Our measurement of a higher metal abundance supports this claim, and points toward a dual origin for the Stream, with two interwoven strands of material, one pulled out of the SMC about 2 billion years ago, and another pulled out of the LMC more recently.

Ongoing work by our team is investigating the total mass and inflow rate of the Magellanic gas onto the Milky Way, where it will potentially be able to fuel future generations of star formation. However, the gas must first survive the perilous journey through the hot Galactic corona, which can evaporate passing gas clouds. This survivability is being tested with computer simulations.

This Month’s Featured Author

Dr. Brian Williams received his B.S. from Florida State University in 2004 and his Ph.D. from North Carolina State University in 2010. He was a NASA Postdoctoral Fellow at NASA Goddard Space Flight Center for three years, after which he worked as a research scientist at NASA GSFC with Universities Space Research Association. He arrived at STScI in February of 2017, and is currently a Support Scientist in the Science Mission Office. His research interests include supernovae and supernova remnants, shock physics and particle acceleration, and dust in the interstellar medium.